Acknowledgements This research has been supported by grants from USDA NRI, North American Carbon Program, NSF MRI Program, Northern Arizona University Mission Research Program/McIntire-Stennis, Science Foundation Arizona, and Arizona Water Institute. MULTIYEAR ANALYSIS OF THE EFFECTS OF WILDFIRE AND THINNING ON ECOSYSTEM CARBON FLUXES OF PONDEROSA PINE FORESTS Thomas Kolb, Sabina Dore, Mario Montes-Helu School of Forestry, Northern Arizona University before after Undisturbed Thinned Burned 35 km Flagstaff, AZ, USA ARTICLES ABOUT THIS PROJECT Amiro, B.D., A.G. Barr, et al Ecosystem carbon fluxes after disturbance in forests of North America. Journal of Geophysical Research 115, G00K02, DOI /2010JG Dore, S., T.E. Kolb, et al Carbon and water fluxes from ponderosa pine forests disturbed by wildfire and thinning. Ecological Applications 20: Dore, S., T.E. Kolb, et al Long-term impact of a stand-replacing fire on ecosystem CO2 exchange of ponderosa pine forests. Global Change Biology 14:1–20. Montes-Helu, M., T.E. Kolb, et al Persistent effects of fire-induced vegetation change on energy partitioning and evapotranspiration in ponderosa pine forests. Agricultural and Forest Meteorology 149: Roman, M.O., C.B. Schaaf, et al The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes. Remote Sensing of Environment 113: Sorensen, C.D, A.J. Finkral, et al Short- and long-term effects of thinning and fire on carbon stocks in ponderosa pine stands in northern Arizona. Forest Ecology and Management 261: Sullivan, B.W., S. Dore, et al Evaluation of methods for estimating soil carbon dioxide efflux across a gradient of forest disturbance. Global Change Biology 16: Sullivan, B.W., S. Dore, et al. In review. Snowmelt causes pulse emissions of carbon dioxide from snowpack at a high-elevation site in northern Arizona, USA. Arctic, Antarctic, and Alpine Research. Sullivan, B.W., T.E. Kolb, et al Thinning reduces soil carbon dioxide but not methane flux from southwestern USA ponderosa pine forests. Forest Ecology and Management 255: Sullivan, B.W., T.E. Kolb, et al Wildfire reduces carbon dioxide efflux and increases methane uptake in ponderosa pine forest soils of the southwestern USA. Biogeochemistry: DOI /s Yi, C., D. Ricciuto, et al Climate control of terrestrial carbon exchange across biomes and continents. Environmental Research Letters 5: DOI / /5/3/ Figure 2: Relationship between vapor pressure deficit (VPD) and light-saturated net ecosystem exchange (NEE) at the undisturbed (UND) and thinned (THN) sites before (April– August 2006) and after (April-August ) thinning. Figure 3: Monthly light curves of mean NEE (±SE) of PPFD classes for the undisturbed (UND) and thinned (THN) sites, April-September, The sites are compared for the pre-thinning period in 2006 and after thinning in Figure 1: Annual net ecosystem production (NEP), total ecosystem respiration (TER), and gross primary production (GPP) for the undisturbed (UND), thinned (THN), and burned (BUR) sites in years Negative values represent ecosystem sink; positive values represent ecosystem source. WILDFIRE SITE Burned Site Thinned Site Undisturbed Site SITE LOCATIONS Intensely burned in 1996 (~10,000 ha) Little tree regeneration LAI seasonal max. 0.5 to 0.9 m 2 m -2 (0% trees) Carbon stocks Burning reduced total site C ~ 40%, 10 yrs post-fire due to a decrease tree biomass, shift to coarse woody debris, and decrease in forest floor Current stock ~7200 g C m -2 (46% mineral soil, 35% coarse woody debris) ~90 ha in eddy tower footprint thinned fall 2006 after collection of ~1 year pre-treatment data Thinning targeted small trees and reduced tree density 70%, basal area 35%, and LAI 40% Carbon stocks Thinning reduced total site C ~19% in 1 st post- treatment year, mostly due to decrease in trees Post-thinning stock ~ 9000 g C m -2 (38% mineral soil, 36% trees) Largest trees logged early 1900s Natural regeneration Most mature trees ~90 yrs old Little subsequent management Carbon stocks Current stock ~12,400 g C m -2 (53% trees, 32% mineral soil) Similar soils (Eutroboralf), elevation (~2100 m), topography (~flat) INTRODUCTION Ponderosa pine (Pinus ponderosa) forests in much of the southwestern U.S. have become dense, mainly because of decades of fire suppression, and are now prone to high-intensity fire. Fuel- reduction via tree thinning is being used increasingly to protect such forests from intense burning. To determine the effect of both thinning and intense burning on carbon (C) dynamics we compared ecosystem C exchange using eddy covariance for 4 years ( ) among 3 sites: Undisturbed, dense, control site with no treatments in the last century (UND) Site subject to fuel-reduction thinning (THN) Site subject to intense burning in 1996 (BUR) METHODS We used the same closed-system eddy covariance approach based on an LI-7000 IRGA, CSAT3 Campbell sonic anemometer, and CNR1 Kipp and Zonen radiation sensor at all sites. We acquired and processed the raw data using software designed by Giovanni Manca (Centro di Ecologia Alpina, Italy), which has been shown to provide similar values as those produced using the Ameriflux standard (Aubinet et al., 2000). The software applies coordinate rotation, linear detrending, and corrects for flux losses. The fluxes are quality flagged following steady state and integral turbulence characteristic tests, and precipitation, scalar variances, and spikes. Positive fluxes indicate ecosystem carbon losses; negative fluxes carbon uptake. Missing data were gap-filled using different methodologies, including gap-filling bad quality data or only data with u* below the threshold of 0.2 m s -1 (Dore et al. 2008). Variance among gap-filled estimates was used to estimate standard errors for carbon-exchange values. CONCLUSIONS Carbon sink strength (i.e. annual NEP) of undisturbed, dense ponderosa pine forests of the southwestern U.S. is low relative to more mesic forests and is sensitive to disturbance. Intense burning that converts forests to grasslands reduces C stock (~40%) and coverts land from a C sink to a source for decades due to low GPP when trees do not regenerate. Fuel-reduction via thinning small trees causes a small reduction in C stock (~19%) and a short-term reduction of C sink strength because reduced tree leaf area reduces GPP. But, C sink recovers rapidly in thinned stands due to increase in GPP of remaining trees. Thinning ameliorates the impact of summer drought and high VPD on C uptake. During drought thinned stands can be stronger sinks than undisturbed stands. THN BUR UND MAIN FINDINGS Figure 1: The burned site was a carbon source all 4 years because TER>GPP. The undisturbed site was a C sink all years with wide interannual variation. The thinned site had similar C sink strength to the undisturbed site before thinning, shifted to a weak C source the first year after thinning, and then shifted back to a carbon sink the second and third post-thinning years. In the usually dry year of 2009, the thinned site was a stronger sink than the undisturbed site. Impacts of thinning on NEP were controlled more by GPP than TER. Figure 2: The relationship between 30-min. NEE and VPD was similar for undisturbed and thinned sites before treatment (2006). Thinning reduced NEE at a given VPD in the 1 st post-treatment year (2007). In the 2 nd and 3 rd post-treatment years (2008-9), NEE was greater at the thinned site than the undisturbed site at high VPD. Figure 3: Light-response curves of 30-min. NEE vs. PPFD confirm similar C sink strength for undisturbed and thinned sites before treatment (2006). After treatment (2007-9), C sink was greater at the undisturbed site vs. thinned site during wet periods, but the opposite pattern (thinned>undisturbed) occurred during pronounced dry periods (June 2007, July-Sept. 2009).